Isotope-ratio analysis is used to measure the relative abundance of isotopes (isotope ratio) in a gaseous sample. For instance, it is used for determining the isotope ratios 13C/12C and/or 18O/16O from CO2 e.g. in air. Isotope-ratio analysis is most commonly performed by mass spectrometry (MS) but may also be performed by optical spectrometry.
Gas inlet systems for isotope ratio analysis are well known, especially for use with isotope ratio mass spectrometers. A general review of isotope ratio mass spectrometry and gas inlet systems can be found in Brenna et al, Mass Spectrometry Reviews, 1997, 16, 227-258. Isotope-ratio mass spectrometry usually comprises comparative measurements of isotope ratios of a sample gas and of one or more reference gases of known isotope ratio. Accordingly, isotope ratio MS (IRMS) typically requires at least one sample gas inlet and at least one reference gas inlet.
A popular solution for gas flow management is the so called open split, which comprises a mixing region that is open to the atmosphere. Gases to be analyzed emerge from a line into the mixing region and whilst a large proportion of the gases is lost to the atmosphere as excess a small amount is transferred to a further line. The open split thus vents gas flow in excess of that acceptable by the isotope ratio analyzer. In isotope ratio MS an example of an open split design is known from U.S. Pat. No. 5,424,539, wherein the open split comprises a small glass vial open to the atmosphere with various sample, reference and carrier gas capillaries ending in the vial and a further capillary sampling the mixing region within the vial. The carrier gas is used to dilute the various sample and reference gases to achieve a desired concentration for analysis. However, the design is not as robust as would be desired and excess sample gas is lost from the system in the open split prior to analysis. A significant amount of carrier gas also has to be used. A similar open split design is described in U.S. Pat. No. 5,661,038 and shown in Tobias et al, Anal. Chem. 1996, 68, 3002-3007. The open split concept has been refined for improved performance and automation, for example as shown in U.S. Pat. No. 7,928,369 and WO 2007/112876, wherein the capillaries for supplying gases are provided with drives for movement in and out of the mixing zone. In all, such open splits as described, essentially comprising an array of nested capillaries, are not simple to manufacture and can lack reproducibility in production as well as robustness in use.
A type of open split is used in gas chromatography MS (GCMS) for pressure adaption, although it is not used in isotope ratio MS. Such an open split is the Open-Split Capillary Interface Part No. 113532 from SGE (www.sge.com).
Gas inlet systems configured for autodilution of samples using an open split are also known in the form of the Thermo Scientific GASBENCH and Thermo Scientific CONFLO interfaces for isotope ratio MS (www.thermoscientific.com).
As mentioned above, isotope ratio MS (IRMS) typically requires at least one sample gas inlet and at least one reference gas inlet. The attained measurement precision is typically about 0.05% and accuracy is derived from that precision by use of the reference gas. However, in isotope ratio optical spectrometry (IROS) there is not currently offered an equivalent effective solution for reference and calibration. A system for calibrating the isotope ratio measurements to account for concentration dependence and a delta scale contraction is described in B. Tuzson et al, High precision and continuous field measurements of δ13C and δ18O in carbon dioxide with a cryogen-free QCLAS, Appl. Phys. B (2008), Volume 92, pp 451-458. However, a drawback with the system described in Tuzson et al is that it utilizes a significant number of separate diluted supplies of reference gases of known isotope ratio. Such reference gas/air mixtures are not commonly available when working in the field for example. Furthermore, the system described Tuzson et al does not employ a sample dilution.
Isotope ratio optical spectrometers differ in several aspects from isotope ratio mass spectrometers: IROS requires a higher inflow of sample; an IROS system overall is more compact, which makes it transportable, but which in turn requires extra ruggedness, and this also applies to the gas inlet system; the IROS market is more price sensitive, necessitating simple low-cost solutions. As a consequence, conventional IRMS gas inlet systems are not first choice for use as IROS inlet systems; and a cheaper, more compact and simpler system designed for higher flow rates is required for IROS measurements.
From this background it can be seen that it would be desirable to provide a gas inlet system for an isotope ratio analyzer that is compact and robust, easy and cheap to manufacture, and furthermore allows comparative measurement of isotope ratios of a sample and one or more reference gases. It would also be desirable in an inlet system to reduce the loss of excess sample gas that occurs with conventional open split configurations.
The disclosure has been made against this background in order to try to alleviate one or more of the aforementioned problems as well provide one or more additional advantages as hereafter described.